Wind turbines, particularly megawatt capacity and above, offer high energy efficiency while providing “green” electricity at prices comparable to fossil fuel sources. Large windmills dominate the physical landscape because of two fundamental characteristics. First, the available energy in a wind stream scales like the turbine diameter squared. That is, a 6 meter turbine gathers four times more energy than a 3 meter turbine. Second, the available energy scales like the wind velocity cubed. Mathematically, the theoretical highest efficiency a wind turbine can achieve is the “Betz” limit of 59.3%. This means that no more than 59.3% of the kinetic energy in wind currents can be converted into work. Highly optimized megawatt wind turbines can achieve overall efficiencies of nearly 40%.
There are typically four kinds of wind turbines that incorporate some permutation of the following features: vertical or horizontal rotational axis and drag or lift style blades. Large wind turbines are almost exclusively lift-type horizontal axis wind turbines or “LHAWTs.” While a few single-blade LHAWTs are in use, three-bladed wind turbines are most common and have the form shown in FIG. 1. Although the three blades shown in FIG. 1 add complexity and expense compared to single or double blade types, three blades reduce vibration and speeds to acceptable levels for coupling to a generator which is often mounted in the rectangular nacelle 101 behind the tower.
The blade tips in a typical LHAWT rotate 3 to 10 times faster than the prevailing wind speed so as to extract as much energy as possible during each rotation. If the blades did not spin quickly, most of the air would pass untouched through the large open spaces between the narrow blades, thereby decreasing the turbine's efficiency. The blades are essentially a wing flying through the air, converting linear wind forces to rotary torque through a combination of momentum transfer and Bernoulli pressure differences.
The blades on a typical LHAWT are mounted radially on a central hub, greatly reducing the cost, drag and complexity of additional support structures. Each blade's angle of attack to the wind can be adjusted for optimal efficiency, or to stow the blades in high winds. Notwithstanding, LHAWT designs suffer from a number of deficiencies. Because LHAWT blades rely on lift, they do not operate at low wind velocities (like a plane, they must achieve “take off” speeds before flying). In addition, if the blades become dirty or covered with ice, the lift is dramatically reduced thus decreasing the LHAWT's efficiency. Further, the horizontal blade axle, by rotating in the XZ plane as shown in FIG. 1, must be directly aimed into the wind for highest efficiency and to avoid destructive buffeting.
An alternative approach to the LHAWT is the drag vertical axis wind turbine or “DVAWT,” most notably the Savonius design named after Sigurd J. Savonius. The Savonius turbine as shown in FIG. 2 consists of two or more blades arranged around a vertical axis (denoted as the Y axis in FIG. 2). One side of each blade is scoop-shaped, and the other rounded, though neither side of the blade is particularly aerodynamic. This scooped blade design results in a drag differential between each side of the blade which causes the structure to rotate when placed in a wind current.
If both surfaces of the blade offered equal drag coefficients, the turbine would not rotate because the torques on either side of the tower would balance out. However, in the case of a Savonius wind turbine, the three “scoops” are pulled along by the wind (counter clockwise for the above turbine as indicated by the circular arrow in FIG. 2) because the smoother blade side offers slightly lower air resistance than the scoop side. Efforts have been made to increase the drag difference, mostly by changing the angle of attack of the upstream blade through a series of levers. One disadvantage is that these complex devices are easily damaged in high winds, and the blade adjustment structures themselves add to drag, thereby reducing overall efficiency.
DVAWTs do offer significant advantages over LHAWTs. First, DVAWTs do not have to be aimed into the wind, which is of particular advantage in blustery areas like rooftops or near the ground. Second, DVAWTs start rotating at lower wind speeds than LHAWTs. Third, since the blades are not aerodynamic, dirt, ice and insect gunk are less of a concern. Fourth, DVAWTs rotate essentially at the same speed as the wind, and so tend to be quieter than LHAWTs (whose tips may exceed 0.3 Mach under some conditions).
On the other hand, DVAWTs tend to be a third or less efficient than LHAWTs (perhaps 10% absolute efficiency). The DVAWT's lower overall efficiency is partly due to turbulent losses on the scoops and associated bearings. Also, it is very hard to achieve a large drag differential between the two sides of the blades when all drag sources, including blade support beams and the central axis, are included. Additionally, the lower efficiency is a consequence of 360 degree wind tolerance—meaning that some portion of each blade is always facing away from the wind in a less than optimal direction, ready to catch a change in the wind. DVAWTs are also very difficult to scale to larger dimensions, because the large blades are heavy and require disproportionately stiff supports and foundations.
The remaining two conventional designs include a lift-based vertical axis turbine (LVAWT), often called a “giromill” such as the helical giro shown in FIG. 3. LVAWTs rotate around the vertical (e.g. “Y” axis) shown by the curved arrow in FIG. 3. The helical blades on a typical LVAWT are wing-shaped in cross-section. Like LHAWTs, LVAWT blades spin quickly and do not start rotating in light winds, and like DVAWTs, the blades do not have to be aimed into the wind. But the fragile blade support cage of LVAWTs is hard to scale to large dimensions. Also, one blade is always behind the tower and is thus buffeted by turbulence.
Because LVAWTs do not begin rotating at low wind speeds, a small Savonius windmill is sometimes mounted on the same vertical axis, inside the helix. The Savonius helps start the giromill at lower wind speeds, but at the cost of additional drag and turbulence. LVAWTs can typically achieve 20% efficiency and are popular designs for urban or rooftop installations and are capable of generating a few kilowatts of power in strong winds.
The fourth approach is the drag horizontal axis wind turbine or “DHAWT.” DHAWTs are characterized by vertical blades with a horizontal axle that swings around the horizontal plane (the XZ plane as shown in FIG. 4) to follow the wind. The blades on a DHAWT are flat and non-aerodynamic so as to create enough drag to turn the blades. The large number of blades efficiently intercepts the wind, even at low speeds, but as a consequence turns very slowly, and the blades have to be stowed away from damage in medium strength winds. DHAWTs are typically not used for power generation but are predominately intended to directly power water pumps.